The present disclosure relates generally to apparatus and methods for image sensing, and, more particularly, to a color image sensor comprising pixels having multiple photodetection layers for reproducing color without the need for conventional absorption color filter arrays.
CMOS image sensors have seen tremendous improvement in performance, especially under optimal illumination conditions. But, for low light applications, there is still much room left for improvement, despite the emergence of technologies such as backside illumination and panchromatic filters. Sensors operating in light-starved conditions typically use monochrome sensors since they collect more photons. These sensors are also able to incorporate near-infrared to further increase the collected signal. The lack of color however makes object detection and identification extremely difficult.
The use of absorption color filters is a major limiting factor to a color image sensor's low light performance. Primary color filter arrays typically transmit only about a third of the incident light, while complementary filter arrays tend to have a theoretical maximum transmittance of about two thirds of the incident light.
To make the most use of incident illumination, three pixel layers can be stacked vertically to sample different colors without the need for an absorption color filter as demonstrated by Foveon (see, e.g., U.S. Pat. No. 5,965,875 to Merrill). This is possible because light absorption in silicon is wavelength dependent. The increased complexity associated with implementing readout circuitry for each of the three junctions has limited the growth of this technology. Other challenges such as increased noise, lag and reduced fill factor have hampered the use of this concept for low light imaging.
A compromise which has been explored quite extensively is to stack two pixel layers to reduce the readout complexity and still leverage the wavelength dependent absorption for color sampling. Each pixel outputs two color samples which is insufficient for full color reproduction. Several implementations of two-layer pixels therefore tend to use a color filter array (CFA) to vary the response of pixels in order to generate three or more color samples. See K. M. Findlater, “Buried double junction pixel using green and magenta filters,” in IEEE Workshop on CCDs and Advanced Image Sensors, Japan, 1999; D. J. Tweet, J. J. Lee, J. M. Speigle and D. Tamburino, “2PFC image sensors: better image quality at lower cost,” in IS&T/SPIE Electronic Imaging: Digital Photography V, 7250, 2009; see, also, U.S. Pat. No. 8,106,426 to Tweet, et al. It has been suggested that the use of Green and Magenta checkered filter pattern over the two layer pixels is optimum for both good color reproduction and high SNR (E. R. Fossum, “Investigation of Two-Layer Photodetectors for YSNR10 Improvement in Submicron Pixels,” in International Image Sensor Workshop, Hokkaido, Japan, 2011. The use of color filters however reduces the light collected by the pixels. In order to maximize the light absorption to make these two-layer pixels useful for low light applications, an alternative color sampling method and device is required which eliminates the losses caused by absorption color filters.